1. Field of the Invention
The present invention relates to a wheel of the Francis type for a hydraulic machine, which wheel is designed to pass a forced flow of water. Such a flow drives the wheel in rotation when the machine is a turbine. Such a flow results from said rotation when the machine is a pump. The present invention also relates to a hydraulic machine including such a wheel. The present invention also relates to an energy conversion installation equipped with such a hydraulic machine.
2. Brief Description of the Related Art
Document JP-A-2005 48608 describes a wheel or “runner” for a hydraulic machine of the pump-turbine type. That wheel has an axisymmetric band and a plurality of curved blades that are secured to or integral with the band and each of which has an outside peripheral leading edge and an inside central trailing edge. The connections via which the band is connected to the inside central edge of every other blade are situated on a first circle of large radius, whereas the connections via which the band is connected to the inside central edge of each blade adjacent to said every other blade are situated on a second circle that is of smaller radius. In the outside peripheral region of the wheel, the connections via which the band is connected to the blades having their inside central edges lying on the first circle are situated on the periphery of the band, whereas the connections via which the band is connected to the blades adjacent to those blades and having their central edges lying on the second circle of small radius are situated set back from the periphery of the band. That particular shape for the wheel of JP-A-2005 48 608 is designed to reduce the cavitation arising at the inlet of the turbine, i.e. at the periphery of the wheel, while the hydraulic machine is operating at steady speeds, so as to increase its hydraulic efficiency at steady speeds.
In a prior art hydraulic machine, at transient speeds that occur after starting, or before stopping in turbine mode, the wheel is subjected to rotating and asynchronous radial forces that tend to “skew” it. At transient speeds, those radial forces exerted on the wheel give rise to mechanical stress levels that can be very high and that therefore determine the dimensioning of the components of the hydraulic machine, such as the shaft or the bearings thereof. The cost of those components is thus necessarily increased due to such radial forces.